THIS INVENTION relates to an RF transponder having an antenna and, more particularly, to an RF transponder having an antenna comprising at least two ports.
For certain applications, a two-way radio frequency (RF) communications system might comprise a number of transponders and a reader. The transponders have no direct power source and operate using radiated power from the reader. Hence, this type of arrangement is known as xe2x80x98passive taggingxe2x80x99. Initially, the reader sends out a carrier wave, which may be a continuous wave (CW) or a pulse wave (PW), the purpose of which is to provide a radiated power source for the transponders and to interrogate the transponders as to a specific identification code (ID) carried by each transponder in its internal processor. The reader receives a backscattered version of its own carrier wave. The carrier wave is modified by the identification code such that the information carried by the carrier wave includes the identification of an interrogated transponder, thus allowing the reader to identify the transponder. Once this has been done, the reader establishes a protocol for communication between itself and the transponder. This communication takes the form of the reader sending a digitally modulated control signal which the transponder demodulates.
The transponder has three tasks. These are to rectify AC power derived from the carrier wave, to modulate the backscattered response to the initial carrier wave and to demodulate the control signal from the reader. The transponder has an antenna. If all of these functions were to be carried out using the same antenna and by a shared port of the antenna it would be very difficult to achieve optimum performance as different parameters are required for each of the functions to perform optimally.
Improved performance can be achieved by using two ports on the same antenna, where one port is for the modulator and demodulator, and the other solely for the rectifier. Thus, either at least two antennae are required, or a single antenna with multiple ports is needed.
If two antennae are used, the so-called dual-antenna solution (as is the case with most of the current work in this area), problems arise due to the small size of the transponders. The two antennae need to be isolated from each other (ideally half of a signal wavelength apart) in order not to interfere with each other, and on a transponder of limited size it is often not possible to place the antennae sufficiently far apart (see FIGS. 5 and 6 of the accompanying drawings). This approach thus leads to an oversized transponder, and can also reduce the space available for other components.
Another solution is to use a single antenna with dual ports. However, for good antenna gain performance, a good quality substrate is required, which increases the cost of producing the transponder. Known antennae using this approach incorporate microstrip patch antennae with dual ports, and operate these ports at the same frequency. The antenna gain performance of such a setup using a good quality substrate is not perturbed by microstrip transmission line power feed due to the nature of the electromagnetic fields in the microstrip patch.
It is an object of the invention to seek to provide an RF transponder having an antenna which incorporates a plurality of ports, without requiring the use of an expensive substrate, and without incurring significant loss of gain performance for any of the ports.
Accordingly, one aspect of the invention provides an RF transponder having an antenna comprising at least two ports operable to receive respective signals such that, in use, the signals in combination establish at least two current nulls on the antenna, each of the ports being located substantially at a current null.
Advantageously, the antenna is a loop.
This loop, preferably, is formed as a coplanar structure.
Conveniently, the antenna loop is rectangular.
Advantageously, the number of current nulls is two.
Preferably, the length of the perimeter of the antenna is substantially equal to two or more half wavelengths of at least one of the signals.
Conveniently, the signals are 90xc2x0 out of phase with each other.
Advantageously, coplanar stripline is provided for feeding the signals to the antenna.
Preferably, the antenna has two ports.
A further problem which dictates the minimum size of conventional passive transponders is that the magnitude of the demodulated signal is not. sufficient to be read by digital circuits on the transponder. Comparators, which are relatively large and consume substantial amounts of power, are required to boost the demodulated signal, and this increases the size and the power consumption of the transponder.
It is a further object of the present invention to provide a rectifying circuit which can output a demodulated signal of sufficient magnitude to be read directly by conventional digital circuits.
Accordingly, A further aspect of the present invention provides a rectifying circuit containing two ports, each port being operable to have an oscillating voltage applied thereacross, a first rectifying means being connected to the first port and being constructed such that, in operation, the output of the first rectifying means is substantially constant and a second rectifying means being connected to the second port and being constructed such that, in operation, the output of the second rectifying means fluctuates in phase with the applied voltage.
Preferably, the rectifying circuit has an output comprising the outputs of the first and second rectifying means combined with one another.
Conveniently, the outputs of the first and second rectifying means are combined by summation.
Advantageously, the combined output is bridged by a DC storage capacitor to provide a DC power source.
Preferably, the first rectifying means performs a voltage multiplication effect.
Conveniently, the second rectifying means performs a voltage multiplication effect.
Advantageously, the first rectifying means comprises first and second diodes and first and second capacitors, the capacitance of the second capacitor being sufficiently large to provide a smoothing effect that, in operation, renders the output of the first rectifying means approximately constant.
Preferably, the second rectifying means comprises third and fourth diodes and third and fourth capacitors, the capacitance of the fourth capacitor being sufficiently small that, in operation, the output of the second rectifying means fluctuates in phase with the applied oscillating voltage.
Another aspect of the invention provides an RF transponder including a rectifying circuit according to the above, in which the combined output of the first and second rectifying means comprises a demodulated signal for the transponder and the output is further bridged by a DC storage capacitor to provide a DC power source for the transponder.